Introduction

Multicolor super-resolution fluorescence microscopy is a valuable method for observing interactions between intracellular structures and biomolecules. A straightforward way to achieve multi-color imaging is to introduce a third channel on the widely available nanoscopes with 775 nm STED wavelength relied on using large Stokes shift (LSS) labels. However, LSS dyes for live-cell labeling have still been missing. In this paper, S. W. Hell, and his co-workers developed of LSS fluorophores capable of penetrating intact plasma membranes of living cells. Besides, they should be compatible with the popular 775 nm STED wavelength. Following the recent report by the Klań group [1] and an earlier work of Wu and Burgess,[2] they had identified 9-aminopyronin scaffold as a promising LSS analog of rhodamine.